For charging battery with a lower input voltage by linear charger, lower dropout voltage is necessary and therefore, the die area of the power switch in the linear charger has to be larger for lower dropout voltage. The conventional linear chargers are made by PMOSFET-base current source. To reduce the die area, NMOSFET is better than PMOSFET by the higher mobility. However, the controller and driver are complex in NMOSFET than in PMOSFET by the current sensing accuracy and power consideration. FIG. 1 is a popular structure in linear chargers, for example in U.S. Pat. Nos. 6,522,118, 6,700,324 and 6,407,532, by using PMOSFET, in which a linear charger 100 has a pair of common gate charging PMOSFET 106 and sensing PMOSFET 108 to act as a current source. The charging PMOSFET 106 has a source coupled to a power input terminal 102 and a drain coupled to a power output terminal 104 for supplying a charging current Ic. The sensing PMOSFET 108 also has its source coupled to the power input terminal 102, so the charging PMOSFET 106 and the sensing PMOSFET 108 have a same gate-source voltage Vgs and thereby produce source-drain currents Ic and Is proportional to each other. If a current setting/sensing circuit 112 virtually shorts the drain of the charging PMOSFET 106 to the drain of the sensing PMOSFET 108, the source-drain current Is of the sensing PMOSFET 108 will reflect the charging current Ic more accurately. A resistor 114 is coupled between the current setting/sensing circuit 112 and a ground terminal GND, to receive the source-drain current Is of the sensing PMOSFET 108, to generate a sensed voltage VS to represent the charging current Ic. A loop controller 110 controls the gate voltage VG of the charging PMOSFET 106 in accordance with the output voltage VOUT and the sensed voltage VS, to control the charging current Ic.
When the linear charger 100 is connected with a lower input voltage VIN to charge a battery, it is desired a lower voltage drop of the charging PMOSFET 106 for less power loss. Since the voltage drop of the charging PMOSFET 106 is equal to the product of its on-resistance and current, it is possible to reduce power loss by lower on-resistance or lower charging current Ic of the charging PMOSFET 106. However, while the charging speed depends on the magnitude of the charging current Ic, lower charging current Ic will result in longer charging time of the battery. On the other hand, while the on-resistance of the charging PMOSFET 106 depends on the size of its channel, lower on-resistance requires larger die area, which causes more costs and is disadvantageous to circuit shrinking. For these reasons, it is impossible to further reduce die area and manufacturing costs of the linear charger 100 without increasing power loss and prolonging charging time.
With a same die area, compared with PMOSFET, NMOSFET possess higher mobility and thereby lower on-resistance. In case the current source of a linear charger is implemented with NMOSFETs, instead of PMOSFETs, the die area can be significantly reduced without increasing power loss and prolonging charging time. Unfortunately, NMOSFETs and PMOSFETs have different driving schemes. For instance, when there is no voltage applied to the gate, a PMOSFET is on while an NMOSFET is off. For the charging PMOSFET 106, the charging current Ic can be supplied as long as the gate voltage VG is lower than the input voltage VIN. Nevertheless, if an NMOSFET replaces the charging PMOSFET 106, it will not supply any charging current Ic unless the gate voltage VG is higher than its source voltage, i.e. the output voltage VOUT. Therefore, in some cases where the gate voltage VG is lower than the source voltage VOUT, the NMOSFET will not be active and thus the linear charger will not operate. Obviously, if the PMOSFETs 106 and 108 are directly replaced by NMOSFETs, the controller and driver would be necessarily complicated in view of the accuracy of current sensing and the variation of supply voltage VIN.